function d=cal_dist(c1,c2,pars)
CENTER_X=pars(1);
CENTER_Y=pars(2);
DX=pars(3);
DY=pars(4);
FOCALLENGTH=pars(5);
FOCALLENGTH_POW2=FOCALLENGTH*FOCALLENGTH;
tempa = (c1(1) - CENTER_X) * DX;
tempb = (c1(2) - CENTER_Y) * DY;
denominator = sqrt(tempa * tempa + tempb * tempb + FOCALLENGTH_POW2);
v1 = -tempa / denominator;
v2 = -tempb / denominator;
v3 = FOCALLENGTH / denominator;

tempa = (c2(1) - CENTER_X) * DX;
tempb = (c2(2) - CENTER_Y) * DY;
denominator = sqrt(tempa * tempa + tempb * tempb + FOCALLENGTH_POW2);
u1 = -tempa / denominator;
u2 = -tempb / denominator;
u3 = FOCALLENGTH / denominator;
d=double(acos((u1 * v1 + u2 * v2 + u3 * v3)/ (sqrt(u1 * u1 + u2 * u2 + u3 * u3) ...
                * sqrt(v1 * v1 + v2 * v2 + v3 * v3))) * 180 / pi);